Chemical vapor deposition involves directing one or more gases containing chemical species onto a surface of a substrate, typically a flat wafer, so that the chemical species react and form a deposit on the surface. For example, compound semiconductors can be formed by epitaxial growth of the semiconductor material on a crystalline wafer. Semiconductors referred to as III-V semiconductors commonly are formed using a source of a Group III metal such as gallium, indium, aluminum, and combinations thereof and a source of a Group V element such as one or more of the hydrides or of one or more of the Group V elements such as NH3, AsH3, or PH3, or an Sb metalorganic such as tetramethyl antimony. In these processes, the gases are reacted with one another at the surface of a wafer, such as a sapphire wafer, to form a III-V compound of the general formula InxGayAlzNAAsBPCSbD where X+Y+Z=approximately 1, and A+B+C+D=approximately 1, and each of X, Y, Z, A, B, C and D can be between 0 and 1. In some instances, bismuth may be used in place of some or all of the other Group III metals.
In certain processes, commonly referred to as a “halide” or “chloride” process, the Group III metal source is a volatile halide of the metal or metals, most commonly a chloride such as GaCl2. In another process, commonly referred to as metalorganic chemical vapor deposition or “MOCVD,” the Group III metal source is an organic compound of the Group III metal as, for example, a metal alkyl.
One form of apparatus which has been widely employed in chemical vapor deposition includes a disc-like wafer carrier mounted within the reaction chamber for rotation about a vertical axis. The wafers are held in the carrier so that surfaces of the wafers face in an upstream direction within the chamber. While the carrier is rotated about the axis, the reaction gases are introduced into the chamber from a flow inlet element upstream of the carrier. The flowing gases pass downstream toward the carrier and wafers, desirably in a laminar plug flow. As the gases approach the rotating carrier, viscous drag impels them into rotation around the axis, so that in a boundary region near the surface of the carrier, the gases flow around the axis and outwardly toward the periphery of the carrier. As the gases flow over the outer edge of the carrier, they flow downwardly toward exhaust ports disposed below the carrier. Most commonly, this process is performed with a succession of different gas compositions and, in some cases, different wafer temperatures, to deposit plural layers of semiconductor having differing compositions as required to form a desired semiconductor device. Merely by way of example, in formation of light emitting diodes (“LEDs”) and diode lasers, a multiple quantum well (“MQW”) structure can be formed by depositing layers of III-V semiconductor with different proportions of Ga and In. Each layer may be on the order of tens of Angstroms thick, i.e., a few atomic layers.
Apparatus of this type can provide a stable and orderly flow of reactive gases over the surface of the carrier and over the surface of the wafer, so that all of the wafers on the carrier, and all regions of each wafer, are exposed to substantially uniform conditions. This, in turn promotes uniform deposition of materials on the wafers. Such uniformity is important because even minor differences in the composition and thickness of the layers of material deposited on a wafer can influence the properties of the resulting devices.
Considerable effort has been devoted in the art heretofore to development of flow inlet elements for use in apparatus of this type. Commonly, the flow inlet element has inlets for the reactive gases dispersed over an active, gas-emitting area approximately equal in size to the wafer carrier. Some of these flow inlet elements carry the first reactive gas, such as a mixture of a Group V hydride, whereas others carry the second reactive gas, such as a mixture of a metal alkyl and a carrier gas. These inlets may be formed as tubes extending parallel to the axis of rotation, the inlets are distributed over the downwardly-facing or downstream surface of the flow inlet element. Considerable effort has been devoted in the art heretofore to arranging the inlets in symmetrical patterns. Typically, the first gas inlets are provided in a pattern which has radial symmetry about the axis of rotation of the wafer carrier, or which has at least two perpendicular planes of symmetry crossing one another at the axis of rotation. The second gas inlets have been provided in a similarly symmetrical pattern, interspersed with the first gas inlets. The flow inlet element commonly incorporates complex channel structures for routing the gases to the tubular inlets. Moreover, because the wafers typically are maintained at a high temperature as, for example, about 500° C. to about 1200° C., the flow inlet element must be provided with coolant channels. The coolant channels carry a circulating flow of water or other liquid and thus maintain the temperature of the flow inlet element relatively low, so as to limit or preclude premature reaction of the gases. As disclosed, for example, in U.S. Published Patent Application No. 20060021574 A1, the disclosure of which is hereby incorporated by reference herein, a flow inlet element may be provided with additional structures for discharging flows of a carrier gas devoid of reactive species.
The carrier gas flows isolate the reactive gas flows from one another while the gases are in the vicinity of the flow inlet element. The gases do not mix with one another until they are remote from the flow inlet element. Moreover, discharging the carrier gas flows limits or prevents recirculation of the reactive gases as they exit from the flow inlet element. Thus, the reactive gases do not tend to form undesired deposits on the flow inlet element. As described, for example, in commonly assigned U.S. Published Patent Application No. 20080173735 (now U.S. Pat. No. 8,152,923), the disclosure of which is hereby incorporated by reference herein, recirculation of the discharged gases in the vicinity of the flow inlet element may be reduced by providing blade-like diffusers projecting downstream from the surface of the flow inlet element to guide the gas flows.
Typically, the inlets are constructed and arranged to provide uniform flow velocity away from the flow inlet element over the entire active region of the flow inlet element, i.e., the entire area where the inlets are arranged. In some cases, the gas inlets for a particular gas may be partitioned into two or more zones, as for example, a first zone near the axis of rotation and a second zone remote from the axis. These two zones may be provided with separate gas channels so that the flow rates of the first gas can be controlled independently in the two regions. For example, in one common arrangement, the inlets for a first gas, such as a Group V hydride, are arranged in an array covering most of the flow inlet surface, whereas the inlets for a second gas, such as a Group III alkyl, are arranged in one or more narrow strips extending generally radially with respect to the central axis. In such a system, a portion of a strip disposed remote from the axis supplies the second gas to a ring-like portion of the wafer carrier having a relatively large area, whereas a portion of the same strip near to the axis supplies the gas to a ring-like portion of the wafer carrier having a smaller area. To provide equal flux of the second gas per unit area of the wafer carrier, it has been common to zone the second gas inlets to provide unequal rates of discharge of the second gas per unit length along the strip. For example, the inlets near the axis may be supplied with a gas mixture having a relatively low concentration of the second gas, whereas the inlets remote from the axis may be supplied with a more concentrated gas mixture. Such zoning adds to the complexity of the system.
Despite all of these developments, still further improvement would be desirable.